David Haas , Nov 16, 2012; 10:40 a.m.
I can't comment on the quality of the Sigma 1.4 since I haven't used it, but typically lenses are at their sharpest 2 - 3 stops down. Even though the 1.4 or 1.8 does provide very shallow dof (desireable for both sports and portraits), the lenses are actually sharpest at 2.0 or 2.8...
I have the Nikon 85 f1.8 - the older version (not the newest G) and can state that is an excellent performer in low light and portraits. Plus it is light enough to carry all day.
The Sigma 70-200 f2.8 is a good lens, but it is bigger and heavier - plus for volleyball you really won't be using the long end much. And for soccer - depending upon the field size - you may find it too short. My daughter is 9 and plays on smaller fields than standard, for I think 3 more years, then will move up to the larger "regulation" fields. Typically when I shoot soccer on reg fields I go with the Sigma 150-500.